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2003 Monsoon 335 Overheating


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I give up....(searching has led to only one thread, and it was a catted motor that was fixed by a factory bulletin).

2003 Monsoon 335 with 980 hours.  Never had an overheating issue before this season.  Boat runs great (compression 150-160 in all cylinders), and performs as well as it always has.  At cruising speeds (skiing, etc.) temp stays right at 160.  As soon as you drop back to idle speeds, the temp rapidly climbs to 200, and the alarm sounds.  The temperature then returns back to 160 (within  1 minute, or so).  

Using my infrared thermometer, and shooting at the front of the heads, I was getting readings around 160 on the driver side, but 200 on the passenger side.  Knocked out the freeze plugs, and flushed quite a bit of "debris" out of the motor.  Just took it back out this morning.  Same overheating at idle, and gets up to 200 on both sides :lol::Frustrated:.

So far I have:

1. Replaced Johnson pump (original was leaking out of the rear).  In doing so, I replaced the impeller as well.  Verified correct part/model number.

2. Replaced water pump on front of motor.  When that was off, I poked around a bit to clean out what I could reach.  Debris in cooling passages is a mixture of metal particles (magnet pulls it out) and black "gunk" that stains your skin.  

3. Replaced both the temp sensor, and sender units.

4. Replaced thermostat.  Tried running a 140 degree unit that I had as well.  No difference.

5.  Knocked out front, and lower rear freeze plugs, and flushed out the cooling passages.  

6. Pulled both exhaust manifolds/risers and cleaned out the water passages.  Replaced all four gaskets.

7. Pulled various hoses while the boat is running to be sure water is flowing through them.

8. Switched out the impeller this morning, just to see if it was as simple as that.  Nope.

This is driving me nuts!!!

Any other ideas or things to check? 

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39 minutes ago, 85 Barefoot said:

go to the pressure side of the tranny cooler, remove hose, check screen for debris.

That was the problem with my brother inlaws LXI this weekend.  In all the years they have owned it they have never had an issue with clogging the trans cooler with weeds - their lake is full of them.   This time the trans cooler inlet was covered with a mat of stringy weeds.

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I appreciate the responses!

1 hour ago, 85 Barefoot said:

go to the pressure side of the tranny cooler, remove hose, check screen for debris.

I had checked it previously, but just checked again.  All clear.

1 hour ago, justgary said:

Re-seat all of the hoses from your raw water pickup to the pump.  You may well have a small leak that is sucking air.  

Just went through and slightly repositioned/tightened all hose clamps.  Thinking of bypassing the heater to simplify things.

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1 hour ago, powbmps said:

I appreciate the responses!

I had checked it previously, but just checked again.  All clear.

Just went through and slightly repositioned/tightened all hose clamps.  Thinking of bypassing the heater to simplify things.

Was going to suggest heater.  You installed an aux pump, right?  You sure that didn't crack or anything?

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29 minutes ago, UWSkier said:

Was going to suggest heater.  You installed an aux pump, right?  You sure that didn't crack or anything?

No water leaks where the aux pump is, but I went ahead and bypassed the heater.  

Hopefully give it another shot in the morning. 

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Still no luck.  Went out this morning and the temp was jumping to 200 when dropping back to idle.  As an experiment I removed the thermostat.  Other than running a little cool, no issues at all.

So....am I correct in assuming there is an issue with the thermostat?  It was overheating with the old 160 and new 140, so I don't see how that would be the case.

Another thing I am trying to comprehend....the thermostat location between the two halves of the housing vs. under the bottom half.  The top outlets directly supply the exhaust manifolds only.   Doesn't that mean the water is not going to the manifolds until the temperature reaches 160, and the thermostat opens?

Link to the thermostat diagram: https://imgur.com/LgNYwly

 

Edited by powbmps
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@powbmps - sounds like a potential weak or leaky impeller that basically loses its pumping capability at low speeds.  By leaky I mean internally not pumping efficiently.  As noted this also happens with an air leak, weeds in the system or some bypass as all posts refer to.  I have witnessed a strainer not seated properly does this also.  You might pull the screen for a quick test.

Looks like you have covered all the suggestions so I’ll ponder it some more.  Have you looked at the sending unit itself, is it in a T fitting and if yes coupled to what?  I assume the pyrometer temps and gauge temps are matching and consistent with each other.  
 

System plumbing should have a bypass that feeds exhaust manifolds until tstat opens.  The offshore guys drill a couple of holes in the stat as a bleed.

Edited by Woodski
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2 hours ago, Woodski said:

@powbmps - sounds like a potential weak or leaky impeller that basically loses its pumping capability at low speeds.  By leaky I mean internally not pumping efficiently.  As noted this also happens with an air leak, weeds in the system or some bypass as all posts refer to.  I have witnessed a strainer not seated properly does this also.  You might pull the screen for a quick test.

Looks like you have covered all the suggestions so I’ll ponder it some more.  Have you looked at the sending unit itself, is it in a T fitting and if yes coupled to what?  I assume the pyrometer temps and gauge temps are matching and consistent with each other.  
 

System plumbing should have a bypass that feeds exhaust manifolds until tstat opens.  The offshore guys drill a couple of holes in the stat as a bleed.

The impeller and Johnson pump are new (was having the overheating issue prior to replacing both).  I have removed my heater and shower from the equation to simplify things.  No in-line strainers or screens.  

Both sensors (or is it sensor and sender?) have been replaced.  They are threaded directly into the intake manifold, to either side of the thermostat.  Alarm starts to sound when the temps hit 200.  Gauge shows 200, and the digital readout on the speedometer agrees (shows temp only when the alarm is sounding).  Seem to be consistent with each other.

You are right about the bypass.  It is not shown in that Indmar diagram, but there is a small hole in front of the thermostat that constantly lets water through.  

Timing was slightly advanced, so I adjusted that before going out today.   

Once again, I appreciate the responses!

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Not likely because the opposite should be happening, but is the raw water intake hose ok and not collapsing?

If it were a British sports car I would call it a feature and say turn up the radio, but ... 😱

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If you're getting an air leak somewhere, presumably you'd be able to find it with the engine pressurized.  What if you fab up a connection so you can connect your garden hose directly to your intake hose and turn it on?  You'll get a small bit of water leaking out of the exhaust through the bypass in the Tstat housing, but you should also be able to tell if you have something else leaking air in as I'd expect water to seep out, no?  I'm thinking it's something weird like a small crack in a brass heater fitting or something.

Exhaust leakage like in @Bozboat video.

 

Edited by UWSkier
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  • 4 months later...

Just reviving this for winter, as I never solved the problem.

So far I have:

Replaced leaking Johnson pump. No difference.

Tried 2 new impellers. No difference.

Replaced water pump. No difference.

Tried 140, and two 160 thermostats (boat would overheat when coming back to idle, but not under load).

Tried no thermostat (boat ran fine, but did not get above ~120).

Checked compression, all good.

Knocked out freeze plugs and flushed out any debris in block.  Cleared out quite a bit of Florida mud, but only made temperatures more consistent on both sides of motor.

Pulled manifolds and risers to be sure no blockage.

Pumped water into motor to be sure no leaks, and good flow through hoses and manifolds, also checked intake screen on bottom of boat.

Ran it on fake-a-lake in the driveway with 160 thermostat and no overheating issues.  This got me pretty psyched until...

Took it back out on lake, and it overheated within a few minutes of 1,000 rpm operation.  Cools back down with throttle, and then climbs right back up when coming back to idle.

Ended up running the boat most of the summer with a flat piece of metal, with a 3/8" hole drilled in the middle, in place of the thermostat.  Boat idles right at 160, but cools off to 120-140 when underway.  At those temps, it is obviously not making any heat for these cold mornings.  However, no affect on performance, or top speed.

Just looking for any other advice or things to try.  Thank you!

(Any way clogged fuel injectors could make the motor run hot, but not affect overall performance?)

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Are you using an indmar specific t stat? I replaced mine and found an after market version that matched exactly.   I had over heating issues.   Replaced it with an indmar one and everything ran great.  I never could figure out why the other didn't work.  Looked the same and had the same specs but it didn't work as well.  

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5 hours ago, PURDUEskier said:

Are you using an indmar specific t stat? I replaced mine and found an after market version that matched exactly.   I had over heating issues.   Replaced it with an indmar one and everything ran great.  I never could figure out why the other didn't work.  Looked the same and had the same specs but it didn't work as well.  

Interesting.  The original Indmar I pulled out doesn't seem to fully open on the stovetop until ~200 degrees.  Replaced it with a Sierra version which seems to be open closer to 180, but still overheats.  I just ordered the Indmar version from Bakes.

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5 hours ago, justgary said:

This still sounds just like an air leak on the suction side of your raw water.  The pressure from your fake lake can keep it from sucking air. 

This makes sense, but I believe I have ruled this out.  The only things before the raw water pump are the intake screen, shutoff valve and (in my case) the Y-pipe to the heater.  Earlier this summer I replaced the Y-pipe with a straight fitting to eliminate that from the equation, checked all of the connections, and still had the same issue.    

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8 hours ago, powbmps said:

Just reviving this for winter, as I never solved the problem.

So far I have:

Replaced leaking Johnson pump. No difference.

Tried 2 new impellers. No difference.

Replaced water pump. No difference.

Tried 140, and two 160 thermostats (boat would overheat when coming back to idle, but not under load).

Tried no thermostat (boat ran fine, but did not get above ~120).

Checked compression, all good.

Knocked out freeze plugs and flushed out any debris in block.  Cleared out quite a bit of Florida mud, but only made temperatures more consistent on both sides of motor.

Pulled manifolds and risers to be sure no blockage.

Pumped water into motor to be sure no leaks, and good flow through hoses and manifolds, also checked intake screen on bottom of boat.

Ran it on fake-a-lake in the driveway with 160 thermostat and no overheating issues.  This got me pretty psyched until...

Took it back out on lake, and it overheated within a few minutes of 1,000 rpm operation.  Cools back down with throttle, and then climbs right back up when coming back to idle.

Ended up running the boat most of the summer with a flat piece of metal, with a 3/8" hole drilled in the middle, in place of the thermostat.  Boat idles right at 160, but cools off to 120-140 when underway.  At those temps, it is obviously not making any heat for these cold mornings.  However, no affect on performance, or top speed.

Just looking for any other advice or things to try.  Thank you!

(Any way clogged fuel injectors could make the motor run hot, but not affect overall performance?)

Did you replace the circulation water pump or just the Johnson impeller raw water pump?

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1 minute ago, Bozboat said:

Did you replace the circulation water pump or just the Johnson impeller raw water pump?

Yes sir.  The passages were getting pretty narrow in the original, but unfortunately the new one didn't solve the problem. 

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3 hours ago, powbmps said:

This makes sense, but I believe I have ruled this out.  The only things before the raw water pump are the intake screen, shutoff valve and (in my case) the Y-pipe to the heater.  Earlier this summer I replaced the Y-pipe with a straight fitting to eliminate that from the equation, checked all of the connections, and still had the same issue.    

Even the cover plate on your raw water pump can suck air if it is loose in the right place.  Unless you have a blockage in your engine, I can't think of another reason you would have those symptoms. 

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